Nitrogen fluoride includes four types, namely, azine fluoride (N.sub.3 F), dinitrogen difluoride (N.sub.2 F.sub.2), nitrogen trifluoride (NF.sub.3) and dinitrogen tetrafluoride (N.sub.2 F.sub.4). Of these, nitrogen trifluoride (NF.sub.3) is a representative nitrogen fluoride. Nitrogen trifluoride is a physically and chemically stable gas at ordinary temperature but under conditions such as heat ultraviolet ray and plasma discharge, the nitrogen trifluoride is decomposed to release active fluorine. Therefore, this gas is used, for example, as a gas for etching or cleaning in semiconductor processes.
When nitrogen trifluoride gas is employed for the above-described uses, the exhaust gas evolved contains nitrogen fluoride as the decomposition product of nitrogen trifluoride, such as dinitrogen tetrafluoride (N.sub.2 F.sub.4) and dinitrogen difluoride (N.sub.2 F.sub.2), and undecomposed nitrogen trifluoride. These nitrogen fluorides are highly toxic and have a large global warming potential coefficient (GWP value). Accordingly, nitrogen fluorides present in the exhaust gas must be decomposed into harmless substances before they are released into the atmosphere.
Several techniques have heretofore been proposed for decomposing nitrogen fluorides contained in the exhaust gas. The methods proposed are roughly classified into (1) decomposition by a reactive agent (catalyst) and (2) decomposition by combustion. The former method is advantageous in that the apparatus can be theoretically simplified in construction as compared with the latter method, and a large number of simple and easy techniques have been proposed. In principle, these techniques achieve the object by converting fluorine components in the nitrogen fluoride into easily decomposable fluoride gases such as SiF.sub.4, BF.sub.3 and WF.sub.6, into stable solid fluorides such as AlF.sub.3 and FeF.sub.3, or into fluorocarbon gases such as CF.sub.4. However, the method of converting nitrogen fluoride into easily decomposable gaseous fluorides is not necessarily simple in view of the treatment process as a whole because a secondary treatment of the fluoride gas is necessary. Similarly, the method of converting nitrogen fluoride into stable solid fluorides is not necessarily a simple and easy method because when a metal oxide is used, NO.sub.z is generated as by-product and a special treatment such as reduction of NO.sub.x is necessary. Furthermore, the method of converting nitrogen fluoride into stable gaseous fluorocarbon is not preferred because the fluorocarbon itself is a substance having a high global warming potential coefficient.